Liquid crystal display devices utilize optical anisotropy and dielectric anisotropy of liquid crystal substances. The liquid crystal display devices are widely used in tabletop calculators, word processors, and television sets, including watches, and demand for the display devices is in a trend to increase year by year. Liquid crystal phase exists between solid phase and liquid phase, and is divided broadly into nematic phase, smectic phase, and cholesteric phase. Among them, nematic phase is most widely employed for display devices at present. On the other hand, while many display modes were devised up to date, three types of twist nematic (TN) mode, super twist nematic (STN) mode, and thin film transistor (TFT) mode have now become main currents. Properties required of liquid crystal substances (liquid crystalline compounds) for these various liquid crystal display devices are different depending on their uses, but any of the liquid crystal substances is required to be stable against outside environmental factors such as moisture, air, heat, and light; to exhibit liquid crystal phase at a temperature range as wide as possible with room temperature being its center; to be in a low viscosity; and to have a low driving voltage. However, no liquid crystal substances which satisfy such requirements at the same time by a single compound have been found.
With respect to liquid crystal substances used for liquid crystal display devices, it is an actual circumstance that several kind or several tens kind of liquid crystalline compounds, and further several kind of liquid non-crystalline compounds when necessary, are usually mixed to produce liquid crystal compositions and used for display devices, in order to adjust such physical properties as dielectric anisotropy (.DELTA..epsilon.), optical anisotropy (.DELTA.n), viscosity, and elastic constant ratio K.sub.33 /K.sub.11 (K.sub.33 : bend elastic constant, K.sub.11 : splay elastic constant) of liquid crystal compositions to most suitable ones required of each display device. Accordingly, liquid crystalline compounds are required to be excellent in miscibility with other liquid crystal compounds, and recently in particular required to be excellent in the miscibility even at low temperatures from the requirement of being used in various environments.
Meanwhile, active matrix mode, especially thin film transistor (TFT) mode is extensively adopted in recent years as display mode, for example, for television sets and viewfinders from the aspect of display performances such as contrast, display capacity, and response time. Also, STN mode which has a large display capacity and display devices of which can be produced by comparatively simpler methods and at a lower cost than those of active matrix mode from the structural factor of display devices is largely adopted in displays, for example, for personal computers.
Recent development in these fields is being progressed while placing stress on
(a) downsizing of liquid crystal display devices into a portable size as shown by the development of small size TV sets and notebook size personal computers both of which are characterized by being in small size, light, and thus portable; and PA1 (b) production of liquid crystalline compounds and liquid crystal compositions having a low driving voltage, that is, a low threshold voltage from the viewpoint of withstand voltage of IC, in the aspect of liquid crystal material. PA1 1 cell thickness can be reduced since the electrode exists on the substrate only at one side, PA1 2 By reduction of production cost can be expected since cell thickness can be reduced, and PA1 3 By distance between electrodes is maintained constant, can be mentioned in addition to the fact that the viewing angle is expanded. PA1 1) effect of raising clearing point of liquid crystal compositions, PA1 2) effect of reducing viscosity of the compositions, and PA1 3) effect of preventing the dielectric anisotropy of the compositions from lowering (or preventing the threshold voltage of the compositions from raising), PA1 X.sup.1, X.sup.2, and X.sup.3 independently represent single bond, 1,2-ethylene group, vinylene group, --COO--, --CF.sub.2 O--, or --OCF.sub.2 -- provided that at least one of X.sup.1, X.sup.2, and X.sup.3 represents --COO--, --CF.sub.2 O--, or --OCF.sub.2 --; PA1 ring A.sup.1, ring A.sup.2, ring A.sup.3, and ring A.sup.4 independently represent trans-1,4-cyclohexylene group CH.sub.2 group on which ring may be replaced by oxygen atom, or 1,4-phenylene group one or more hydrogen atoms of which may be replaced by fluorine atom or chlorine atom; and m and n are 0 or 1 PA1 provided that when X.sup.1, X.sup.2, or X.sup.3 represents --COO--, then at least one of ring A.sup.2, ring A.sup.3, and ring A.sup.4 represents 2,3-difluoro-1,4-phenylene group, and Y.sup.1 represents an alkoxy group; PA1 when m=n=0 and X.sup.1 represents --COO--, then ring A.sup.1 represents 1,4-phenylene group at least one hydrogen atom in which group is replaced by fluorine atom; PA1 when m=1, n=0, X.sup.1 represents single bond or 1,2-ethylene group, and X.sup.2 represents --COO--, then ring A.sup.2 represents 1,4-phenylene group at least one hydrogen atom in which group is replaced by fluorine atom; PA1 when m=n=1, X.sup.2 represents --COO--, and X.sup.1 represents single bond or 1,2-ethylene group, then ring A.sup.2 represents 1,4-phenylene group at least one hydrogen atom in which group is replaced by fluorine atom; PA1 when m=n=1, X.sup.3 represents --COO--, and X.sup.1 and X.sup.2 independently represent single bond or 1,2-ethylene group, then ring A.sup.3 represents 1,4-phenylene group at least one hydrogen atom in which group is replaced by fluorine atom; and PA1 when m=n=0 and X.sup.1 represents --CF.sub.2 O-- or --OCF.sub.2 --, then ring A.sup.1 or ring A.sup.4 represents trans-1,4cyclohexylene group, or 1,4-phenylene group at least one hydrogen atom in which group is replaced by chlorine atom or fluorine atom; PA1 when m+n=1, X.sup.2 or X.sup.3 represents --CF.sub.2 O--, ring A.sup.2 or ring A.sup.3 represents 1,4-phenylene group, and A.sup.4 represents 1,4-phenylene group at least one hydrogen atom in which group may be replaced by a halogen atom, then Y.sup.1 represents an alkyl or alkoxy group, and PA1 each element which constitutes this compound may be replaced by its isotope. PA1 Y.sup.2 represents fluorine atom, chlorine atom, --OCF.sub.3, --OCF.sub.2 H, --CF.sub.3, --CF.sub.2 H, --CFH.sub.2 OCF.sub.2 CF.sub.2 H, or --OCF.sub.2 CFHCF.sub.3 ; PA1 Z.sup.1 and Z.sup.2 independently represent 1,2-ethylene group, vinylene group, 1,4-butylene group, --COO--, --CF.sub.2 O--, --OC.sub.2 --, or single bond; PA1 ring B represents trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, or 1,4-phenylene group hydrogen atom of which may be replaced by fluorine atom; PA1 ring C represents trans-1,4-cyclohexylene, or 1,4-phenylene group hydrogen atom of which may be replaced by fluorine atom; and PA1 each atom which constitutes these compounds may be replaced by its isotope. PA1 Y.sup.3 represents CN group or --C.tbd.C--CN; PA1 ring D represents trans-1,4-cyclohexylene, 1,4-phenylene, pyrimidine-2,5-diyl, or 1,3-dioxane-2,5-diyl group; PA1 ring E represents trans-1,4-cyclohexylene, 1,4-phenylene group hydrogen atom of which may be replaced by fluorine atom, or pyrimidine-2,5-diyl group; PA1 ring F represents trans-1,4-cyclohexylene or 1,4-phenylene group; PA1 Z.sup.3 represents 1,2-ethylene group, --COO--, or single bond; PA1 L.sup.3, L.sup.4, and L.sup.5 independently represent hydrogen atom or fluorine atom; PA1 a, b, and c are independently 0 or 1; and PA1 each atom which constitutes these compounds may be replaced by its isotope. PA1 ring G, ring I, and ring J independently represent trans-1,4-cyclohexylene, pyrimidine-2,5-diyl, or 1,4-phenylene group at least one hydrogen atom of which may be replaced by fluorine atom; PA1 Z.sup.4 and Z.sup.5 independently represent --C.tbd.C--, --COO--, --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, or single bond; and PA1 each atom which constitutes these compounds may be replaced by its isotope. PA1 ring K represents trans-1,4-cyclohexylene or 1,4-phenylene group; PA1 Z.sup.6 and Zindependently represent --CH.sub.2 CH.sub.2 ---, --CH.sub.2 O--, or single bond; and PA1 each atom which constitutes these compounds may be replaced by its isotope.
It is known that threshold voltage (V.sub.th) can be expressed by the following equation (H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81): EQU V.sub.th =.pi.(K/.epsilon..sub.0.DELTA..epsilon.).sup.1/2
In the equation described above, K is an elastic constant and .epsilon..sub.0 is a dielectric constant in vacuum. As will be seen from this equation, two methods, that is, a method of increasing dielectric anisotropy (.DELTA..epsilon.) and a method of lowering elastic constant can be considered to lower the threshold voltage. However, since actual control of elastic constant is very difficult, it is an actual situation that liquid crystal materials having a high dielectric anisotropy (.DELTA..epsilon.) are ordinarily used to cope with the requirements. With the facts described above for a background, development of liquid crystalline compounds having a high dielectric anisotropy (.DELTA..epsilon.) has actively been conducted.
Almost all liquid crystal compositions currently used in display devices for TFT mode are composed of fluorine type liquid crystal materials. This is because (i) a high voltage holding ratio (V.H.R.) is required in TFT mode from the viewpoint of construction of the devices, (ii) the materials have to be small in the temperature dependency, and (iii) materials other than fluorine type can not meet these requirements. As fluorine type materials for low voltage, the following compounds are heretofore disclosed: ##STR2##
in the structural formula described above, R represents an alkyl group.
Whereas it is reported that either compounds (a) and (b) have several fluorine atoms at a terminal of the molecule and exhibit a high dielectric anisotropy, it is known that their clearing point (NI point) is low and viscosity is a comparatively high. Among the persons skilled in the art, it is empirically known that there are inversely proportional and direct proportional relations between the number of substituted fluorine atom and the clearing point, and between the number of substituted fluorine atom and the viscosity, respectively, although it is not simple. Accordingly, it is difficult to attain the required clearing point and viscosity (response speed) when liquid crystal compositions are produced only a series of these compounds. With the object of offsetting this defect, a viscosity decreasing agent represented by the following compounds is usually added in liquid crystal compositions. ##STR3##
in the structural formula described above, R and R' represent an alkyl group.
Whereas compounds (c) have a comparatively low viscosity, their clearing point is not sufficiently high to offsetting the low clearing point of liquid crystal compositions comprising the liquid crystalline compounds for low voltage described above. In order to meet the requirement, a comparatively large amount is necessary to be added, but characteristics of liquid crystal compositions are lost in this case. Accordingly, compounds (c) are unsuitable as material to solve the problems described above. Whereas compounds (d) have a sufficiently high clearing point, their viscosity is remarkably high since they have a four ring structure. Thus, the increase in the viscosity is unavoidable when the compounds (d) are added in liquid crystal compositions. Besides, since compounds (d) themselves have smectic phase, when liquid crystal compositions prepared by adding the compounds were left at a low temperature, smectic phase some times develops in the liquid crystal compositions. Accordingly, compounds (d) are unsuitable to solve the problems described above, either. Further, since any one of compounds (c) and (d) has an extremely low dielectric anisotropy value, when it is added to liquid crystal compositions for low voltage having a large dielectric anisotropy value as described above, the compound considerably lower the dielectric anisotropy of the liquid crystal compositions. As the result, the threshold voltage of the compositions raise, and thus, the compound is not preferable to solve the problems.
In the meantime, researches to overcome narrow viewing angle which is only one defect of liquid crystal panel of TFT display mode were actively conducted recent years, and many results of the researches are reported at lectures in academic society and disclosed in patent publications. As an example of the step for the improvement, the following method is disclosed. For instance, in Laid-open Japanese Patent Publication Nos. Hei 4-229828 and Hei 4-258923, a method is disclosed in which the viewing angle is improved by disposing a phase difference film between a pair of polarizing plates and a TN type liquid crystal cell. In Laid-open Japanese Patent Publication Nos. Hei 4-366808 and Hei 4-366809, a method is disclosed in which two layers liquid crystal system using a layer of chiral nematic liquid crystal as phase difference film is adopted. However, either methods described above are insufficient in improvement of the viewing angle and also had such problems that production cost is high and liquid crystal panels become heavy.
As a new method to solve the problems, In-Plane Switching (IPS) driving has recently come to attract public attention (R. Kiefer et al., JAPAN DISPLAY '92, 547 (1992); G. Baur, Freiburger Arbeitstagung Flussigkristalle, Abstract No. 22 (1993)). As characteristics in the structure of liquid crystal panels of the IPS drive, the fact that whereas an electrode is disposed on both upper and lower substrates, respectively, in conventional liquid crystal panels, a comb-shape electrode is disposed on the substrate only at one side in the IPS drive, and the fact that the direction of major axis of liquid crystal molecules is all the time in parallel to the substrates in the IPS drive can be mentioned. As advantages of the IPS drive,
In order to actualize high speed response and low voltage drive in the IPS drive, the liquid crystalline compounds to be used are required to have a low viscosity and a high negative dielectric anisotropy. Also, as another example of attempts to improve the narrow viewing angle, a method which utilizes a vertical orientation of liquid crystal molecules and is disclosed in Laid-open Japanese Patent Publication No. Hei 2-176625 can be mentioned. One of the characteristics of this method is the use of liquid crystal compositions having a negative dielectric anisotropy. Meantime, as a conventional compound having a high negative dielectric anisotropy, the following compounds are disclosed in the patent publication shown below: ##STR4##
in the structural formula described above, R represents an alkyl group.
Compounds (e) (Japanese Patent Publication No. Sho 61-26899) are reported to have 2,3-dicyano--1,4-phenylene group in their partial structure and exhibit a high negative anisotropy. However, since they have cyano groups, their dependency of voltage holding ratio on temperature is large and viscosity is remarkably high, and thus the compounds can not be used as liquid crystal material for IPS drive utilizing TFT display mode. As described above, compounds having characteristics which are necessary for actualizing the high speed response and low voltage driving in the IPS driving, in a well balanced condition with each other, have not yet been known.